text: Considering the photothermal mechanism alone, the photoacoustic signal is useful in measuring the light absorption spectrum, particularly for transparent samples where the light absorption is very small. In this case the ordinary method of absorption spectroscopy, based on difference of the intensities of a light beam before and after its passage through the sample, is not practical. In photoacoustic spectroscopy there is no such limitation. the signal is directly related to the light absorption and the light intensity.  Dividing the signal spectrum by the light intensity spectrum can give a relative percent absorption spectrum, which can be calibrated to yield absolute values. This is very useful to detect very small concentrations of various materials. Photoacoustic spectroscopy is also useful for the opposite case of opaque samples, where the absorption is essentially complete. In an arrangement where a sensor is placed in a gaseous phase above the sample and the light impinges the sample from above, the photoacoustic signal results from an absorption zone close to the surface. A typical parameter which governs the signal in this case is the "thermal diffusion length", which depends on the material and the modulation frequency and ordinarily is in the order of several micrometers. The signal is related to the light absorbed in the small distance of the thermal diffusion length, allowing the determination of the absorption spectrum. This allows also to separately analyze a surface that is distinct from the bulk. By varying the modulation frequency and wavelength of the probing radiation one essentially varies the probed depth, which results in the possibility of depth profiling  and photoacoustic imaging, which discloses inhomogeneities within the sample. This analysis includes also the possibility to determine the thermal properties from the photoacoustic signal.
  Recently, the photoacoustic approach has been utilized to quantitatively measure macromolecules, such as proteins. The photoacoustic immunoassay labels and detects target proteins using nanoparticles that can generate strong acoustic signals. The photoacoustics-based protein analysis has also been applied for point-of-care testings.
  Another application of the photoacoustic effect is its ability to estimate the chemical energies stored in various steps of  a photochemical reaction. Following light absorption photophysical and photochemical conversions occur, which store part of the light energy as chemical energy. Energy storage leads to less heat evolution. The resulting smaller photoacoustic signal thus gives a quantitative estimate of the extent of the energy storage. For transient species this requires the measurement of the signal in the relevant time scale and the capability to extract from the temporal part of the signal the time-dependent heat evolution, by proper deconvolution. There are numerous examples for this application. A similar application is the study of the conversion of light energy to electrical energy in solar cells. A special example is the application of the  photoacoustic effect in photosynthesis research.